1. Field of the Invention
This invention relates to a focal-plane shutter for digital still cameras which is designed to actuate a first blade and a second blade in turn in the same direction when photographing is performed and to expose an imaging surface through a slit provided by both blades.
2. Description of Related Art
In focal-plane shutters used in digital still cameras, some types of shutters are known in accordance with the specifications of cameras. One of them, like a focal-plane shutter used in a film camera, is equipped with a first blade and a second blade, which are actuated in turn at predetermined timing so that an imaging surface is exposed continuously from one side thereof to the other through a slit provided by the blades.
Digital still cameras using shutters constructed as mentioned above are divided into two classes, those in which an optical finder is provided as in the film camera so that the imaging surface is covered by at least one of the first blade and the second blade, except for the case where photographing is performed, and those, having finders referred to as electronic view finders, in which the entire surface in an exposure aperture is exposed to light from an object so that an image of the object can be observed through a monitor, even when photographing is not performed. Thus, the latter camera is such that, in photography, the first blade is moved to the position (a cocked position) where the imaging surface is covered, and then an exposure operation is performed.
In the film camera, the area of the imaging surface is usually larger than that of the digital still camera. The film camera always has the optical finder. Due to the arrangement of an optical system and the placement of a film, each of the first blade and the second blade is composed of a plurality of blade components to achieve compactness of a shutter unit. However, in the digital still camera, particularly provided with the electronic view finder, each of the first blade and the second blade need not necessarily be constructed with a plurality of blade components, and is also known to have a single blade component. An example of a shutter constructed from such a viewpoint is set forth in Japanese Patent Preliminary Publication No. Hei 11-326992.
As is well known, a digital still camera equipped with an electronic view finder is such that when the power of the camera is turned on, light from an object strikes the imaging surface even in the case where photographing is not performed at once, and a finder function is obtained. Thus, when the camera is not used and the power is turned off, it is convenient for the next use of the camera to keep a state where the imaging surface is not covered. In an image sensor such as a CCD, however, when the imaging surface is exposed to the light for a long time period, its performance of photoelectric conversion is degraded even though the image sensor is not energized. Hence, it is desired that when the power is turned off, the imaging surface is prevented from striking the light.
In order to do so, it is conceivable that a lens cap is provided and is mounted and dismounted each time the camera is used. However, this is very cumbersome, and in addition, there is the fear that the lens cap may be lost. Thus, in order to avoid such problems, it is also conceivable that a lens barrier is provided. In a camera constructed so that its lens barrel protrudes from the camera body, however, the structure of the lens barrier is not mechanically easy and is unfavorable for design and cost. In contrast to this, for a digital camera equipped with a shutter, when the camera is not used, the shutter can be held closed and hence the above problems are solved.
In the camera equipped with the shutter, however, in order to obtain the function described above, it is necessary that when the power is turned on, the shutter is brought into an open state from a closed state, and it is closed once in photography to perform an opening and closing operation and is brought again into the open state after photography. Moreover, when the power is turned off, there is the need to perform the operation that the shutter is brought into the closed state from the open state. In a lens shutter which does not require a very high shutter speed, this operation is achieved with comparative ease when the opening and closing operation is performed directly by forwarding and reversing a motor.
However, in a focal-plane shutter having a first blade and a second blade, constructed so that, in order to achieve a high shutter speed in an exposure operation, a driving member for the first blade and a driving member for the second blade which are connected to the first blade and the second blade, respectively, are actuated in turn by individual driving springs, it is not easy that these driving members and cocking members for moving the driving members to cocked positions are constructed so that the opening and closing operation mentioned above is favorably performed. Furthermore, such a structure requires spaces for incorporating the first blade and the second blade which are narrower than those set forth in Hie 11-326992.
It is, therefore, an object of the present invention to provide a focal-plane shutter for digital still cameras in which the first blade and the second blade are actuated in turn by individual driving springs as driving sources in photography to expose the imaging surface through a slit provided by these blades, and when the power of a camera is turned off, the imaging surface is covered by the first blade or the second blade, while when the power is turned on, the imaging surface is exposed, except for photography, and in which areas for incorporating the first blade and the second blade are small.
In order to achieve the above object, the focal-plane shutter for digital still cameras of the present invention includes a first blade covering an exposure aperture only immediately before an exposure operation is performed, after release, but removed from the exposure aperture in other cases even when the power of a camera is turned off; a driving member for the first blade having an iron piece member, connected to the first blade and actuated by the biasing force of a driving spring for the first blade in an exposure operation; an electromagnet for the first blade for attracting and retaining the iron piece member of the driving member for the first blade by the supply of current immediately after the release and actuating the driving member for the first blade by the driving spring for the first blade when the current is interrupted; a cocking member for the first blade whose actuation is controlled between its initial position and cocked position by a first motor, actuating the driving member for the first blade against the biasing force of the driving spring for the first blade when actuated from the initial position to the cocked position, and actuated from the cocked position to the initial position only immediately after the current is supplied to the electromagnet for the first blade; a second blade covering the exposure aperture when the power is turned off and immediately after the exposure operation is completed, but removed from the exposure aperture in other cases; a driving member for the second blade having an iron piece member, connected to the second blade and actuated by the biasing force of a driving spring for the second blade in the exposure operation; an electromagnet for the second blade for attracting and retaining the iron piece member of the driving member for the second blade by the supply of current immediately after the release and actuating the driving member for the second blade by the driving spring for the second blade when the current is interrupted; and a cocking member for the second blade whose actuation is controlled between its initial position and cocked position by a second motor, actuating the driving member for the second blade against the biasing force of the driving spring for the second blade when actuated from the initial position to the cocked position, and actuated from the cocked position to the initial position only immediately after the current is supplied to the electromagnet for the second blade and when the power of the camera is turned off.
Further, the focal-plane shutter for digital still cameras of the present invention includes a first blade covering an exposure aperture immediately before an exposure operation is performed and when the power of a camera is turned off, after release, but removed from the exposure aperture in other cases; a driving member for the first blade having an iron piece member, connected to the first blade and actuated by the biasing force of a driving spring for the first blade in an exposure operation; an electromagnet for the first blade for attracting and retaining the iron piece member of the driving member for the first blade by the supply of current immediately after the release and actuating the driving member for the first blade by the driving spring for the first blade when the current is interrupted; a cocking member for the first blade whose actuation is controlled between its initial position and cocked position by a first motor, actuated from the initial position to the cocked position immediately before the exposure operation is performed and when the power of the camera is turned off, after the release, to actuate the driving member for the first blade against the biasing force of the driving spring for the first blade and actuated from the cocked position to the initial position when the power of the camera is turned on and immediately after the current is supplied to the electromagnet for the first blade; a second blade covering the exposure aperture only immediately after the exposure operation is completed, but removed from the exposure aperture in other cases even when the power is turned off; a driving member for the second blade having an iron piece member, connected to the second blade and actuated by the biasing force of a driving spring for the second blade in the exposure operation; an electromagnet for the second blade for attracting and retaining the iron piece member of the driving member for the second blade by the supply of current immediately after the release and actuating the driving member for the second blade by the driving spring for the second blade when the current is interrupted; and a cocking member for the second blade whose actuation is controlled between its initial position and cocked position by a second motor, actuating the driving member for the second blade against the biasing force of the driving spring for the second blade when actuated from the initial position to the cocked position, and actuated from the cocked position to the initial position only immediately after the current is supplied to the electromagnet for the second blade.
When the focal-plane shutter for digital still cameras mentioned above is designed to have a first sensor for detecting the working position of the cocking member for the first blade to stop the rotation of the first motor and a second sensor for detecting the working position of the cocking member for the second blade to stop the rotation of the second motor, this is favorable in the case where each of the first and second motors is a DC motor.
Further, when the focal-plane shutter for digital still cameras mentioned above is designed to have a first switch for synchro flash operated at the final stage of the exposure operation of the driving member for the first blade by the driving member for the first blade and a second switch for synchro flash operated at the initial stage of the exposure operation of the driving member for the second blade by the driving member for the second blade and to perform flash photographing by a proper choice between these switches, the flash photographing can be widely performed.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment when taken in conjunction with the accompanying drawings.